Chapter 8 - Metamorphism

Question 1

Comment on environments of metamorphism

Answer 1

When rocks get hot, they get cooked, and altered.
Area around igneous intrusions at depth
Area around volcanic environments (shallow)
Or just where it gets deep, and highly pressurized

Question 2

What is shock metamorphism

Answer 2

Shockwave-related deformation and heating during impact events
-ex; when a meteorite hits

Question 3

Define metamorphic rock

Answer 3

A metamorphic rock is a result of a transformation of a pre-existing rock. The original rock is subjected to very high heat and pressure, which cause obvious physical and/or chemical changes. Examples of these rock types include marble, slate, gneiss, schist.

Question 4

What are the two basic groups of metamorphic rocks;

Answer 4

Foliated rocks: generic term for all sorts of minerals phyllosilicate, leafy, thin layer silicate minerals that have sheets
Non-foliated

Question 5

Examples of foliated metamorphic rocks

Answer 5

Slate, phyllite, schist, gneiss, amphibole, migmatite

Question 6

Examples of non-foliated metamorphic roks

Answer 6

Marble, quartzite, greenstone, hornfels, anthracite

Question 7

Define foliation:

Answer 7

Parallel flakes of mica forming a planar texture

Question 8

What are the types of foliation? (include veryfine –> fine)

Answer 8

Phyllitic (phyllite) [very fine]
Slaty (slate) [fine]
Schistositycoarse crystals

Question 9

How is vertical slaty cleavage (type of folation) made?

Answer 9

Directional stress of a bedrock, you get syncline/anticline, and then when you look at a cross section of bed, you see horizontal layers (slaty foliation)

Question 10

What are the two types of pressure exerted on rocks

Answer 10

Confining stress (pressure in all 3 dimensions from all sides)

Directional stress (applying pressure only on 2 sides)

Question 11

What are the results of confining/uniform stress on rock?

Answer 11

Rock retains its shape, recrystallization under confining stress
minerals will retain their shape, but they are still unreformed
-smallest crystals disappear
-crystals get bigger b/c smaller crystals dissolve into fluids that are in there, then they recrystallize in another crystal and that crystal gets bigger

Question 12

What are the results of directional/differential stress on rock?

Answer 12

applies pressure on 2 sides.

• mineral grains, if they have a bladey aspect to them, will align themselves perpendicular to largest stress applied (think lots of pens and you push on them, they will align perpendicular to your stress direction)
• smaller grains dissolve or go away, and the remaining ones realign themselves (the same minerals are lined up together)

Question 13

Causes and agents of foliation and of metamorphism

Answer 13

Pressure
-confining (uniform) stress
-directional (differential) stress
Heat + Fluid
-recrystallization
-new mineral formation

Question 14

Metamorphic grade defn

Answer 14

Informal indication of the amount or degree of metamorphism

-based on appearance of key minerals

Question 15

Index minerals + metamorphic grade

Answer 15

CMB GSKS
Chlorite (low)
Muscovite (low-med)
Biotite(med-low)
Garnet(med)
Staurolite(med)
Kyanite(med)
Sillimanite(high)
Feldspar (unaltered --> high grade)
Quartz (unaltered --> high)
Clay (unaltered)

Question 16

Important example of lithification + diagenesis

Answer 16

Mud to mudstone via lithification

Mudstone to shale via Diagenesis

Question 17

Temperature ranges of grades

Answer 17

low (200-300)
med (300-500)**
high(500-800)

Question 18

Transformations/metamorphosis of shale through increased pressure and temperature
(these are all the metamorphic rocks in order from low grade to high grade)
Also, what can you say about their grain size?

Answer 18

Shale->slate->phyllite->schist->gneiss->migmatite->melting
low grade - shale/slate/phyllite
med grade - phyllite/schist/gneiss
high grade - migmatite
The higher the grade, the larger/coarser the grain size

Question 19

What grade are micas?

Answer 19

Intermediate

Question 20

What happens to clay minerals after increased pressure and temperature?

Answer 20

clay –> micas –> garnet

intermediate grade

Question 21

What mineral groups are inert to the metamorphic process?

Answer 21

Quartz - may recrystallize but remains the same

Feldspar - change subtly in composition, but same feldspar

Question 22

So high grade is 500-800 degrees. What happens after 800 degrees?

Answer 22

Melting minerals

Question 23

Compare grain sizes as rocks get metamorphasized (increased temp and pressure)

Answer 23

Coarser and coarser

Question 24

Explain overall metamorphic process for minerals and rocks

Answer 24

Minerals: new mineral formation as increased depth and pressure, some are index minerals, some are inert to the high pressure/temp and stay the same with a few changes (quartz, feldspar)

Rocks: Start out as shale, and metamorphasize as the temperature and pressure increase

Question 25

Describe grain sizes of;
Slates
Phyllite
Schist
Gneiss

Answer 25

Slates - platy, dense, impermeable. Cant see composition with naked eye

Phyllite - little more course crystalline (higher in metamorphic step). We can start to see foliation with naked eye, tends to break into leafy sheet
-tend to break into leafy sheets

Schist: muscovite, the mica flakes off when its on your hand

• discrete flakes of muscovite
• cosmetic purposes

Gneiss- coarse crystalline
-discrete bands of feldspars

Migmatite;
anatexis (process of partial melting, b/c so high grade that the temp is very high)
-partially liquid partially solid when it was being formed

Question 26

Formation of magma due to heating, and/or water uptake

Answer 26

Anatexis via fractional melting
(process of partial melting)
ex)
-Mineral 1 and mineral 2 in a cup
-after fractional melting, the remaining solid’s pressure causes the rock to “squeeze” the liquid out of the rock
-THIS IS CALLED THE METASOMATIC SOLUTION
-chemical composition of a rock/mineral is altered by partial melting (anatexis)

Question 27

Describe what happens to migmatite wrt temperature (partial melting)

Answer 27

Migmatite –> METASOMATIC SOLUTION –> ANATEXIS

-chemical composition of a rock/mineral is altered by partial melting (anatexis)

Question 28

What happens to limestone when it undergoes metamorphism? Usefullness/limiations of the result?

Answer 28

Limestone + metamorphism = marble

• coarse mosaic of limestone
• Moh’s hardness of marble is soft, so its very good for sculpturing
• limitation: marble dissolves easily with acid rain, cuz limestone(carbonate) dissolves with acid

Question 29

What happens to quartz when it undergoes metamorphism?

Answer 29

Quartzite

• harder
• hard to chisel, but lasts very long

Question 30

Metamorphic facies and their associated temperature - pressure conditions

Answer 30

Metamorphic facies are things like contact metamorphism, volcanic arc, collisional mountain belt, all these occurences (that are based on temperature and pressure)
-they are characterized by mineral assemblages, that were formed under similar temperature and pressure

Question 31

How to use metamorphic facies?

Answer 31

ex) If we find blue schist metamorphic rock, we know its a subtuction zone
ex) If we find amphibolite, we know its contact metamorphism

People map this out, which helps us find things like copper deposits

Question 32

What is contact metamorphism? Where do they occur?

Answer 32

Contact metamorphism at igneous intrusions;

• pillow lava on top of surface
• rock under pillow lava gets fried, creates a fried surface
• rock under fried surface don’t get fried

Question 33

On metamorphic facies, where will you find anatexis?

Answer 33

High temperature, high pressure